mcp(fix[middleware]): Stop error logging from masking the tool result

[tony]

Summary

A failure in the error-logging path could replace the real tool error. ToolErrorResultMiddleware.on_call_tool logged the error before converting it, and _log_error's self.logger.log(..., exc_info=...) call was unguarded — so when the active handler raised while formatting the record, the logging exception escaped the except block and _error_tool_result never ran. The stock fastmcp transform then surfaced a misleading Internal error: … in place of the real failure.

The concrete trigger we hit: a long-lived stdio server whose virtualenv was rebuilt underneath it. fastmcp's RichHandler lazily imports rich.traceback the first time it renders an exc_info record; after the rebuild that import raised ModuleNotFoundError, and every tool call surfaced Internal error: No module named 'rich.traceback' — masking both the cause and the remedy (reconnect the server). The defect is independent of that trigger: any handler failure could mask a real tool error.

The asymmetry that pinpointed it: the error_callback invocation immediately below in _log_error is already wrapped in try/except; the logger.log call on the same path was not.

Changes

• src/libtmux_mcp/middleware.py: wrap the _log_error call in on_call_tool with contextlib.suppress(Exception), guaranteeing _error_tool_result always runs. The suppress is silent by design — logging the failure would re-enter the same broken handler.
• tests/test_middleware.py: parametrized regression test (logging-ok / logging-raises) driving on_call_tool with a call_next that raises a known error and a logger patched to raise; asserts the original error text survives and the rich.traceback message never leaks into the result.

A follow-up worth considering separately (not in this PR): fail fast at startup — eagerly import the traceback dependency or run a one-shot self-check so a broken environment surfaces loudly at launch rather than on the first tool call.

Fixes #66.

Test plan

• New test fails on main (drop the suppress, logging-raises reproduces the ModuleNotFoundError escape) and passes with the fix.
• uv run ruff format . / uv run ruff check . --fix --show-fixes — clean.
• uv run mypy — no issues.
• uv run pytest — full suite green.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.59%. Comparing base (9bdccd1) to head (39a07dc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #67      +/-   ##
==========================================
+ Coverage   84.51%   84.59%   +0.07%     
==========================================
  Files          42       42              
  Lines        2758     2759       +1     
  Branches      370      370              
==========================================
+ Hits         2331     2334       +3     
+ Misses        322      321       -1     
+ Partials      105      104       -1     

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